Project Summary: Immune-checkpoint receptors such as CTLA-4 and PD-1 have been successfully targeted for cancer immunotherapy. Our earlier work has identified V domain Immunoglobulin Suppressor of T cell Activation (VISTA) as an immune-checkpoint protein that critically regulates anti-tumor immunity. Despite this progress, how VISTA inhibits T cell activation is not understood. Our current study has identified a novel VISTA-interacting receptor that is expressed on murine and human T cells and mediates the suppressive effect of VISTA. The overarching goals of this proposal are to define the cellular and molecular mechanisms of this novel T cell coinhibitory complex. Specific aim 1 will dissect the interactions between VISTA and its receptor and establish the relevance of ubiquitin-modifying enzymes to the inhibitory signaling. Aim 2 will examine the role of VISTA and its receptor in regulating tumor-specific T cell responses, utilizing a newly generated conditional knockout mice and novel inhibitors that block this inhibitory receptor complex. The protein expression level of VISTA and its receptor in human cancer tissues will also be examined. Together, these proposed studies will build a foundation for developing novel inhibitors that target the VISTA receptor complex for cancer immunotherapy.